A wealth of genetic information and some biochemical analysis have made the GAL regulon of the yeast Saccharomyces cerevisiae a classic model system for studying transcriptional activation in eukaryotes. Galactose induces this transcriptional switch, which is regulated by three proteins: the transcriptional activator Gal4p, bound to DNA; the repressor Gal80p; and the transducer Gal3p. We showed previously that NADP appears to act as a trigger to kick the repressor off the activator. Sustained activation involves a complex of the transducer Gal3p and Gal80p mediated by galactose and ATP. We solved the crystal structure of the complex of Gal3p-Gal80p with a-D-galactose and ATP to 2.1 Å resolution. The interaction between the proteins occurs only when Gal3p is in a ''closed'' state induced by ligand binding. The structure of the complex provides a rationale for the phenotypes of several well-known Gal80p and Gal3p mutants as well as the lack of galactokinase activity of Gal3p.
Derived from the yeast whole-genome duplication, Saccharomyces cerevisiae GAL1 and GAL3 encode the catabolic enzyme galactokinase (Gal1) and its transcriptional coinducer (Gal3), whereas the ancestral, preduplicated GAL1 gene performed both functions. Previous studies indicated that divergence was primarily driven by changes in upstream promoter elements, and changes in GAL3's coding region are assumed to be the result of drift. We show that replacement of GAL3's open-reading-frame with GAL1's results in an extended lag phase upon switching to growth on galactose with up to 2.5-fold differences in the initial cell masses. Accordingly, the binding affinity of Gal3 to Gal80 was found to be greater than 10-folds higher than that of Gal1, with both a higher association rate (ka) and lower dissociation (kd) rate. Thus, while changes in the noncoding, regulatory regions were the initial driving force for GAL3's subfunctionalization as a coinducer, adaptive changes in the protein sequence seem to have followed.
Virophages are small, dsDNA viruses that can only replicate in a host by co-infecting with another virus. Marine alga are commonly associated with virophage-like elements like Polinton-like viruses (PLVs), which are thought to be ancestors of dsDNA viruses but remain uncharacterized. Here we isolated a PLV that co-infects the alga Phaeocystis globosa with the Phaeocystis globosa virus-14T (PgV-14T). We name this PLV "Gezel-14T" and show that it is phylogenetically distinct from the Lavidaviridae family where all known virophages belong. Gezel-14T co-infection decreases the fitness of its viral host by reducing burst sizes of PgV-14T, yet not enough to spare the cellular host population.Genomic screens show Gezel-14T-like PLVs integrated into Phaeocystis genomes, suggesting these widespread viruses are capable of integration with cellular host genomes. This system presents an opportunity to better understand the evolution of eukaryotic dsDNA viruses as well as the complex dynamics and implications of viral parasitism.
Photochemical dimerization reactions of 1,3-diphenyl-1-propen-3-one (chalcone), 9-acetylanthracene, and 9-(methoxycarbonyl)anthracene as guest molecules in inclusion compounds with 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol were studied. The irradiation of a single crystal containing chalcone was carried out in a single step and resulted in a single crystal containing the photodimer in full occupancy. In the case of the crystal containing 9-acetylanthracene, X-ray diffraction data were collected after irradiation for different periods of time. Only one of the two crystallographically in-
Three co-crystals of the light-stable compound 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) with light-sensitive molecules 1,2-dimethyl-2(1H)-pyridinone (a), 6-methyl-2(1H)-pyridinone (b) and 2-methyl-2(1H)-pyridinone (c) were exposed to UV light. It was found that the molecules undergo molecular flip perpendicular to the molecular plane (rotation of ~180 o ). In the first two co-crystals the light-sensitive molecules are disordered which mean that the space provided for them is larger than needed for ordered molecules. Therefore rotation can take place. Moreover, in I-b the flip is temperature dependent and takes place without exposure to UV light. Crystal structures at 4 different temperatures enable to estimate the activation energy of the flip to be 9.72 kJ/mol. The kinetics of the reaction of I-c was studied at room temperature and revealed a sigmoidal behavior with Avrami exponent of n=0.95(6) that could be explained by the JMAK model for crystal growth. It means that the nucleation rate is constant over time and that the reaction is homogeneous with equal probability to occur in any region of the sample. This could be explained by the fact that the voids where the reaction and the flip take place are isolated from each other.
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